In an optical information recording medium information is written or read by irradiating a recording layer with a light beam through a lens having a focal depth of several micron meters. However, if the recording layer has dust thereon or flaws when handled, it will cause some defects such as bit errors and dropouts. In order to avoid these defects, a so-called air sandwich construction has been proposed which comprises two sheets of disc-shaped plates which are bonded to each other with an adhesive with protrusions provided thereon or spacers interposed therebetween, and at least one of which is a transparent plate having a recording layer provided on the inner surface thereof, as described in U.S. Pat. Nos. 4,074,282, 4,264,911 and 4,353,767. Furthermore, a so-called lamination constructionhas been proposed which comprises two sheets of disc-shaped plates at least one of which is provided with a recording layer, which plates are laminated with each other directly or with a third plate interposed therebetween in such a manner that the recording layer is positioned inside, as described in Japanese patent application (OPI) Nos. 130243/83, 133532/82, 63039/84, 68849/84 and 68850/84.
As an adhesive therefor, there has been commonly used adhesives made of photopolymerizable compounds such as ultraviolet-curing adhesives which take as short period of time, such as several seconds to several minutes, to bond the materials, as described in Japanese patent application (OPI) Nos. 164455/82, 133532/82 and 120243/82, and thus do not require any heating process which might deform the materials.
However, these conventional ultraviolet-curing adhesives are disadvantageous in that they do not exhibit sufficient durability with glass and plastic plates under the circumstances of high temperature and humidity. These conventional ultraviolet-curing adhesive also are unsatisfactory for bonding plates made of acrylic resin or plastic material such as vinyl polychloride and polycarbonate, in that the areas of the materials thus bonded can easily be detached from each other under the slightest deformation of the disc.
In order to improve the adhesivity of such adhesives to the material, it has been proposed that the adherent surface of the material be roughened as disclosed in Japanese patent application (OPI) Nos. 138064/82 and 120243/82. However, this solution is very disadvantageous in that such a roughening process produces dusts which tend to attach to the surface of the material and must be removed therefrom with pure water which has been prepared by a water purifier. Furthermore, the plates thus water-washed must be dried by a clean drier. Thus, this roughening process is disadvantageous in that a costly facility investment and a complicated procedure must be undertaken simply for an improvement in the adhesivity of the adhesive to the material.
Alternatively, it has been proposed that the plates be subjected to discharging treatment or irradiation with ultraviolet rays and then bonded to each other with an electromagnetic radiation-curing adhesive, assuring a greatly improved adhesivity of the adhesive to the material, as disclosed in Japanese patent application (OPI) No. 198548/84. However, this solution is disadvantageous in that it requires a surface treatment process.
Moreover, if an epoxy adhesive is used, a problem arises that amines used as curing agents will deteriorate the recording layer, causing some defects upon recording or reading information. Also, since an epoxy adhesive takes a long time to cure, the plates have to be fixed for a long period of time, until the curing is finished, upon assembly of discs, so that a slightest deviation from the predetermined position can be avoided, which adds to complication of the production process.